"Flash Overlay" is explained on our main page, here. The other features are relatively new, and aren't documented yet:

Quickblock -- allows page-content to be instantly hidden. Just press shift and double-click. Hold down on the last click and swing around to catch multiple elements.
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FastCollapse -- speeds render-time for blocked elements by not collapsing the space they contain. Elements are empty, but visible.

Org:
My bad- I was sleepy when I wrote that. The Dev-page doesn't really qualify as documentation, since the "New Features" list scrolls older entries out.
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We should delete these last two posts now :P